Constant current circuit

Abstract

The constant current circuit includes a constant current generation circuit, a start-up detection circuit configured to detect start-up of the constant current generation circuit, and a clamp circuit configured to output a start-up voltage to the constant current generation circuit. The start-up voltage output from the clamp circuit is a voltage close to gate voltages that are higher than gate voltages of transistors that form a current mirror circuit of the constant current generation circuit, in a state where the constant current generation circuit is operating.

What is claimed is: 
     
       1. A constant current circuit comprising:
 a constant current generation circuit including:
 a resistor connected to a ground terminal; 
 a first current mirror circuit connected to the ground terminal and the resistor, the first current mirror circuit containing a plurality of transistors; and 
 a second current mirror circuit connected between a power supply terminal supplying a power supply voltage and the first current mirror circuit; and 
 
 a start-up detection circuit configured to detect start-up of the constant current generation circuit, and transmit a detection signal; 
 a clamp circuit configured to apply a start-up voltage to the constant current generation circuit, the start-up voltage being a voltage close to the gate voltages of the plurality of transistors in the first current mirror circuit in a state where the constant current generation circuit operates, the start-up voltage being higher than the gate voltages of the plurality of transistors; and 
 a power supply start-up detection circuit configured to detect whether the power supply voltage is applied to the power supply terminal, start up the clamp circuit by detecting the power supply voltage applied to the power supply terminal, receive the detection signal from the start-up detection circuit, and stop the clamp circuit by receiving the detection signal; 
 wherein the start-up detection circuit is configured by a constant current inverter, 
 wherein the constant current inverter contains an output terminal, and 
 wherein the power supply start-up detection circuit includes a logic circuit configured to receive the detection signal from the constant current inverter, a capacitor provided between the power supply terminal and the output terminal, and a current mirror circuit configured to allow a through-current to flow into the capacitor, the through-current being generated when the logic circuit inverts an output signal.